The present invention relates generally to image analysis, and more particularly to the analysis of images of bodily structures, where those images are used for spatial measurements of those structures.
The main arteries, like the aorta, the coronary arteries, the carotid arteries, the renal arteries and the femoral arteries provide the body with blood. Artery stenosis is an abnormal condition characterized by the constriction or narrowing of these vital arteries by a substance known as plaque, that prevents proper circulation of the blood. Plaque is a localized area of arteriosclerosis. Arteriosclerosis is a pathological condition that deposits lipids (fatty substances) and a proliferation of fibrous connective tissue on the inner walls of the arteries. Also anatomical (inherited) restrictions of the arteries exist (for instance because of tortuous vessels) or enlargements of the arteries (aneurysms) that strongly influence the flow of blood or, as in the latter case, could even endanger the life of the patient by spontaneous rupture of the artery concerned.
Standard treatments for these pathological conditions consist of opening up the stenosed arteries by the use of drugs, or when this proves ineffective, by mechanical means. For example, one may force the opening of the artery by inflation of a little balloon inside the stenosed area (“dotter” or “PTA” procedure) often followed by the placement of a metallic frame (“stent”) inside the artery to prevent it from re-occlusion. Stent-like structures can also be used to treat the dangerous local enlargements of arteries. Important for these treatments is to know the exact position, shape, and extent of the stenoses or aneurysms. This will strongly influence the choice of treatment.
An angiography-based diagnosis is the current standard for determining the extend of stenosis. Angiography is a special X-ray procedure that takes pictures (“angiograms”) of blood vessels. This diagnostic technique makes use of a radiopaque contrast medium, which is a chemical substance that strongly absorbs passing X-rays. Angiography is the X-ray visualization of the internal anatomy of the heart and/or blood vessels after the introduction of a radiopaque contrast medium into the blood. The contrast medium may be injected into an artery or a vein or introduced in a peripheral artery through a catheter (hollow tube) inserted in the artery. The radiologist carefully threads it into the blood vessel and guides it to the area to be studied, under continuous X-ray vision. When the catheter reaches the site under investigation, X-ray contrast medium is injected through the catheter and makes the artery with all its irregularities and blockages clearly visible.
The term “angiogram” refers to the radiographic image of a blood vessel produced by angiography. Angiograms have darkened areas that represent open channels in blood vessels caused by the contrast medium blocking the X-rays. Digitizing the resulting images makes it possible to apply image processing techniques to the images. One of these techniques is to perform semi-automatic quantitative measurements of the vascular system such as vessel segment length, diameter, cross-sectional area, and the amount of narrowing of a vessel.
Nowadays most examinations of vessel morphology are done using angiographic images acquired with a monoplane X-ray system. The term monoplane refers to the fact that such a system can only acquire an X-ray image from one direction at a time. In contrast, biplane X-ray systems can acquire images from two directions simultaneously.
Performing quantitative measurements based upon a monoplane angiographic image has two important shortfalls that arise from the near absence of information about the three-dimensional position of the vascular structure. The image is in fact a projection that projects all three dimensional structures onto a two-dimensional plane. The two shortfalls are out-of-plane calibration errors and “foreshortening,” each of which will now be described.
When using a monoplane X-ray image, one must “calibrate” the image in order to make absolute measurements. That is, the relationship between pixel size and real-world size needs to be determined. This can be done by including an object of known size in the recorded image. Problems arise when the object used for calibration is not in the same plane (parallel to the input screen of the image intensifier) as the structure under investigation. This will result in the calibration object being magnified differently than the structure, and hence in an incorrect measurement. In other words, every part of the vessel that is located in a different plane than the calibration object is magnified differently. This error source is called “out-of-plane calibration.”
Generally, because of its intrinsic shape, the structure under investigation typically will not lie exactly in the image plane. If a structure, e.g., a vessel, has a directional component normal to the image plane, the length of a segment of that vessel, when projected on the image plane, will not equal its real three-dimensional length. This phenomenon, leading to errors in length measurements, is called “foreshortening.”
To overcome these problems one needs a more accurate, three-dimensional (3-D) representation of the position and shape of the structure under investigation.
Several methods have been developed that derive three-dimensional information from two digital, two-dimensional images. Stereoscopic digital angiography has been used in the calculation of three-dimensional position and orientation information of vessels (L. E. Fencil et al., Investigative Radiology, December 1987). However, stereoscopic determination of three-dimensional vessel position becomes less accurate if the main direction of the vessel is perpendicular to the direction of the stereoscopic shift. Thus, the reliability of this method in determining three-dimensional vessel structure depends on the orientation of the vessels themselves. This is clearly undesirable.
In U.S. Pat. No. 4,630,203, Szirtes describes a technique for the three-dimensional localization of linear contours appearing in two stereoscopic images. However, this method also suffers from the limitation that the contour must not lie in the direction of the stereoscopic shift. In addition, a separate calibration step is required in this method to determine the three-dimensional locations of the X-ray sources relative to the imaging plane.
Several workers have developed methods to derive three-dimensional structure from two radiographic images that are obtained in exactly orthogonal directions. The prerequisite that the images must be obtained in exactly orthogonal directions is a clear drawback of this method. This may be difficult to achieve in practice. In addition, determination of the positions of vessel segments perpendicular to one of the imaging planes is difficult or impossible with these methods.
To address these problems, a method has been developed that allows calculation of three-dimensional vascular structure from two images obtained at arbitrary orientations (see S. A. MacKay et al., Computers and Biomedical Research, Vol. 15, p. 455, 1982). This method requires a calibration step on an object of known dimensions in the same X-ray system configuration as is used to image the patient. This calibration can be done before or after imaging the patient. This method is also referred to as the “calibrated epipolar geometry” method.
In U.S. Pat. No. 4,875,165, Fencil et al. describe a method that requires a pair of biplane images as input and no calibration step, based upon the theoretical work of H. C. Longuet-Higgings, Nature, Vol. 293, p. 133, 1981. K. R. Hoffmann, U.S. Pat. No. 5,859,922, describes a method using several biplane images with the use of a calibration object. Both methods, however, have the drawback that a biplane X-ray system (that is, an X-ray system capable of making two recordings from different orientations simultaneously) is needed.
Some other methods are aimed at reconstruction of the complete 3-D vascular tree from two or more images, but often require biplane X-ray systems. See for instance U.S. Pat. No. 6,047,080 to Chen et al. and U.S. Pat. No. 6,169,917 to Masotti et al.